The present invention relates to a length or angle measuring apparatus for measuring the relative position of two objects and, more particularly, to a length or angle measuring apparatus which utilizes a scanning unit to scan the graduation of a scale.
Length or angle measuring devices are used, particularly in processing machines to measure the relative position of a tool with respect to a workpiece to be processed, as well as in coordinate measuring machines to determine the position and/or the dimensions of test objects.
In one such length measuring arrangement, for example, a scale is connected with the one object, the graduation of which is scanned by a scanning unit connected with the other object for obtaining periodic scanning signals from which position measuring values are obtained by means of an evaluating arrangement engaged on the outlet side. The values are fed to a position display unit, which displays the values in digital form, and/or to the control arrangement of a processing machine on which the length measuring apparatus is used.
Such length measuring devices are used both as unencapsulated and also as encapsulated measuring devices.
The article "Neuere Entwicklungen bei photelektrischen Langenmess-systemen" (Recent developments in photoelectric length measuring systems) by A. Ernst in "Messen und Prufen/Automatik", July/August 1974, discloses in FIG. 12a an unencapsulated length measuring device in which a scale is fastened to one object by means of several clamping shoes and a scanning unit is fastened to the other object by means of two screws.
German patent specification DE-OS 31 13 962 describes an encapsulated length measuring apparatus in which a scale and a scanning unit are encapsulated in a closed casing for protection against environmental influences. The scale is fastened to the first object. The scanning unit is fastened to an entrainment means, which grips through a sealed longitudinal slit of the casing and is fastened to the second object by means of two screws.
In both the aforementioned length measuring devices the scanning unit is guided, with respect to the scale, by means of the guidance of the two objects to be measured (object guidance). The guidance of the scanning unit to the scale, must occur within certain narrow limits to assure accurate scanning. Therefore, if the attached scanning unit or the entrainment means with the scanning unit is fastened to the object to be measured, very narrow attachment tolerances must be observed. German patent DE-PS 28 10 341 discloses an arrangement which permits coarser attachment tolerances despite these scanning function requirements. A scanning unit is guided on an auxiliary guide which is formed by the scale and/or the casing. The auxiliary guide is in turn connected to the object to be measured. The scanning unit is connected to an entrainment means by means of a coupling element, which is rigid in the measuring direction. The entrainment means is, in turn, fastened to one of the objects to be measured by means of two screws.
Ordinarily, in such arrangements, the scale is fastened to a sliding carriage and the scanning unit to a bed of a processing machine, which are the objects to be measured. After the scanning unit is fastened to the appertaining object to be measured, a reference point is allocated to this object. The reference point corresponds to the scanning point (scanning place) of the scanning element of the scanning unit. The scanning unit scans the graduation of the scale. For example, when a photoelectric scanning unit is used, the graduation may be a scanning graduation field. The reference point may be purely fictitious or be characterized by a mark on the object (bed) to be measured. In a normal thermal condition (for example 20.degree. C.), the starting position of the slide carriage with respect to the bed, the graduation zero point of the scale, the scanning point of the scanning unit and the reference point of the bed all lie in a plane perpendicular to the measuring direction.
While the bed of processing machines generally consists of cast iron, aluminum is the preferred material chosen as the material for the entraining means and synthetic materials are the preferred materials chosen for the scanning unit. This choice of materials results in economical manufacture and reduced weight of the measuring device. Since these differing materials have different thermal expansion coefficients a [object: .alpha.(O)=10.multidot.10.sup.-6 K.sup.-1, entraining means:
.alpha.(M)=21.multidot.10.sup.-6 K.sup.-1 and scanning unit:
.alpha.(A)=35.multidot.10.sup.-6 K.sup.-1 ], relative displacements between the scanning point of the scanning unit, the reference point of the object to be measured and the graduation zero point of the scale may occur with temperature variations relative to the normal thermal condition. Such relative displacements lead to zero-point displacements of the measuring apparatus, which displacements lead to corresponding measuring errors.
For example, temperature increases relative to the normal thermal condition may result from the processing operations on the processing machine. Considerable longitudinal tensile forces in the measuring direction will occur in the scanning unit or in the entraining means because of the rigid fastening, by means of the two screws, of the scanning unit or of the entraining means on the object to be measured. These longitudinal tensile forces arise because the thermal expansion coefficient .alpha.(A) of the scanning unit and the thermal expansion coefficient .alpha.(M) of the entraining means are considerably greater than the thermal expansion coefficient .alpha.(O) of the object to be measured. These longitudinal tensile forces can lead to undefined relative displacements of the scanning unit or of the entraining means at the fastening points on the object. The direction and the magnitude of such relative displacements are not predictable; therefore, the associated zero-point displacement of the measuring apparatus is uncontrollable.
German patent DE-PS 28 53 771 discloses a length measuring apparatus in which a carrier for a scale is connected to an object to be measured by means of a fastening elements disposed at each end of the object. The carrier and the object consist of materials which have different thermal expansion coefficients. While a first end of the carrier for the scale is joined directly with the fastening element, a length compensation element is disposed between the second end of the carrier and the fastening element. The length compensation element provides the second end of the carrier with a translatory degree of freedom in the measuring direction. Therefore, no longitudinal forces arise in the carrier due to thermal length changes of the carrier relative to the object, which forces could act on the scale and impair the accuracy of the measurement.
German patent application DE-OS 32 43 966 teaches a length measuring apparatus in which a carrier, in the form of a casing for a scale, is connected to an object to be measured. The carrier is connected at about its midpoint by a first fastening element and at each end by further fastening elements. The carrier for the scale and the object consist of materials which have different thermal expansion coefficients. While the carrier of the scale is connected directly with the fastening element in the middle, each of the two ends of the carrier is connected to the respective element by means of a length compensation element. The length compensation elements accommodate longitudinal translation of the ends of the carrier in the measuring direction. Thus, with changes in temperature, the position of the scale remains substantially preserved, relative to the object to be measured.
German patent DE-PS 31 06 701 discloses a length measuring device for machine tools, which compensates for the thermal length changes of machine components by means of an expansion bar made of a material with a high thermal expansion coefficient. The expansion bar is fastened to a machine component at one end and to a scale at the other end. The thermal expansion of the expansion bar is equal to that of the machine component, so that the thermal expansion of the machine part is detected and compensated by the length measuring device.
Therefore, in view of the above, it is an object of the present invention to provide length or angle measuring apparatus which will exclude thermally generated relative displacements between the scanning point of the scanning unit and the appertaining reference point of the object to be measured.
It is a further object of the present invention to provide a length or angle measuring device which, with temperature changes, will eliminate relative displacement between the scanning point of the scanning unit and the appertaining reference point of the object to be measured despite different expansion coefficients of the object, the entrainment means and the scanning unit.
It is another object of the present invention to provide a length or angle measuring device which, in a simple manner, will eliminate thermally generated relative displacements between the scanning point of the scanning unit and the appertaining reference point on the object to be measured.
It is still a further object of the present invention to provide a length or angle measuring apparatus which will have greater accuracy, thereby improving the quality of workpieces and considerably lowering the rejection rate of processing machines.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be obtained by means of the instrumentalities and combinations, particularly pointed out in the appended claims.